Separation of aromatic hydrocarbons by extractive distillation with phosphorous tribromide



United States Patem SEPARATION OF AROMATIC HYDROCARBONS 2,736,691 Patented Feb. 28, 1956 BY EXTRACTIVE DISTILLATION WITH PHOS- In extractive distillation processes, the separation of PHOROUS TRIBROM close-boiling aromatic hydrocarbons is based on thechange William T. Nelson, Bartlesville, 0kla., assignor to Phillips in r.e1.ative volatility the hydrocarbons obtained by the Petroleum Company, acorporation of Delaware addition of an entralner so that at least one of the components of the hydrocarbon mlxture can be separated by NO Drawing. Application October 25, 1954, I0 fractional distillation Serial 4645626 The separation process of this invention is usually car- 7 Claims (C1. 202 39 5) ried out as a continuous process, with the ethylbenzene distilling overhead and para-xylene and phosphorus tribromide solvent being recovered as the residual fraction. Tl'liS invention relates to an extractive distillation separa- The phosphorus tribromide is recovered from the botliOIl of aromatic hydrocarbons- In One Of p the toms in the extractive distillation process by a subsequent invention relates to the Separation, y an exflfflctive separation step such as low temperature phase separation, tillation, Of an alkyl'substituted aromatic hydrocarbon fractional distillation, solvent extraction, or the like and from another employing as'an entrainer phosphorus trirecycled to the separation step. bromide which has been found to have a preferential Th separation f phosphorus ib i f h d afiiIIitY for at least one aromatic Component in a miXture carbons by solvent extraction is somewhat difficult be- Of aromatic hydrocarbons boiling in the Same boiling Paint cause of the reactivity of the phosphorus tribromide with range. In another of its aspects, the invention relates to many potential solvents; .however, dimethyl sulfate is an th ep ti of a mono-Substituted aromatic hydrocafeffective solvent for this separation. The separation of b011, from a polyrsubstitllted aromatic hydrocarbon havphosphorus halides from hydrocarbons by solvent extracing a similar or close boiling point, by the employment tion employing dimethyl sulfate is set forth, described and of a phosphorus tribromide entrainer in an extractive disclaimed, in copending application for patent, Serial Numtillation process. In the specific embodiment of the inher 477,840, filed December 27, 1954, by Gardner C. Ray. vention which is considered below, a xylene is removed The proportions of phosphorus tribromide employed in from ethylbenzene, employing phosphorus tribromide as the separation process will vary considerably and depend as extractive distillation entl'ainer. on the concentration, and the nature of the components in Other aspects, the objects, and the advantages of the the mixture to be separated, the purity of the recovered invention are apparent from this disclosure and the approducts desired, and the design and operation of the dispended claims. tillation equipment. Some of the physical properties of In the various ways of treating hydrocarbons, there will the phosphorus tribromide entrainer, used in the example, frequently be encountered mixtures of closely related aroare given in Table I. matic hydrocarbons which are difircult to separate into TABLEI specific components by known procedures. This is especially true of certain mixtures of commonly occurring 0 Physical P p P p 14S tribromide substituted aromatic hydrocarbons such as the Xylenes Molecular Weight 27013 and ethylbenzene. The components in these mlxtures have Boiling point, C 1729 the same number of carbon atoms per molecule and similar Boiling point, F 3432 physical properties so that the separatlon of these hydro- Melting point c 40'0 carbon mixtures into reasonably pure hydrocarbons can- Melting point, P not readlly be obtained by conventional fractlonatlon proc- SpecifiC gravity, 15/4 2.852 esses. Also, other methods of separatlon based on chemical reaction or other methods of separation usually in- EXAMPLE volve physical steps which are expensive, require special A blend of volume percent ethylbenzene and 50 apparatus and reagents, and involve difficult manipulative volume percent paraxylene was subjected to simple batch techniques which achieve only a relatively small yield of 50 distillation in the presence, and in the absence, of phosan impure product. phorus tribromide in a 13 mm. Hypercal column having According to the present invention, it has been found an estimated 40 theoretical trays. The data for these that at least one specific aromatic hydrocarbon can be separations are given in Table II.

TABLE II Distillation of ethylbenzene and para-xylene Charge, ml. Hydro- Cut Bolling iiiifin il iilgifi Emmmer I-Iydro- No. g Distillate, Distillate,

carbon Entrainer Vol. Vol. Percent Blend Percent i asst: as as a; None 48.1 0 1 136.1-136.5 29.2 75.2

- Corrected to 760 mm. Hg.

separated from a mixture of aromatic hydrocarbons having similar boiling point range, and more specifically, that a mono-substituted aromatic hydrocarbon can be separated from at least one poly-substituted aromatic hydrocarbon In a one-stage extractive distillation set-up, the relative volatility of ethylbenzene over para-xylene was found to be 1.09 in the presence of an equal volume of phosphorus tribromide at C. In the absence of phosphorus tribromide, the calculated relative volatility was found to The foregoing data clearly demonstrate that phosphorus tribromide is useful as an extractive distillation entrainer for the separation of close-boiling aromatic hydrocarbons, especially alkyl-substituted aromatic hydrocarbons.

Reasonable variation and some modification are possible within the scope of the foregoing disclosure and the appended claims to the invention, the essence of which is that phosphorus tribromide has been found to be useful as an extractive distillation entrainer for the separation of aromatic hydrocarbons having close boiling points, especially alkyl-substituted aromatic hydrocarbons, substantially as set forth and described herein.

I claim:

1. The extractive distillation separation of close-boiling aromatic hydrocarbons which comprises admixing with said hydrocarbons as an extractive distillation cntrainer phosphorus tribromide and then subjecting the admixture to distillation.

2. The extractive distillation of close-boiling alkyl-substituted aromatic hydrocarbons which comprises admixing same with phosphorus tribromide and subjecting the admixture thus obtained to an extractive distillation.

3. The separation of mono-alkyl-substituted aromatic hydrocarbons from poly-alkyl-substituted aromatic hydrocarbons which comprises admixing said hydrocarbons with phosphorus tribromide as an extractive distillation en- 4 trainer and subjecting the mixture thus obtained to an extractive distillation.

4. The extractive distillation separation of ethylbenzene and paraxylene which comprises admixing with said hydrocarbons phosphorus tribromide and subjecting the said admixture thus obtained to an extractive distillation.

5. The extractive distillation separation of para-xylene from ethylbenzene which comprises admixing approximately 30 volumes of phosphorus tribromide for each 50 volumes of the hydrocarbon admixture with said hydrocarbon admixture and then subjecting the final admixture thus obtained to an extractive distillation.

6. The extractive distillation separation of a mixture of ortho-, meta-, and para-xylenes from ethylbenzene which comprises admixing the said mixture of hydrocarbons with phosphorus tribromide and subjecting the final admixture thus obtained to an extractive distillation.

7. The extractive distillation separation of ethylbenzene from a mixture containing at least one xylene which comprises admixing the said mixture of hydrocarbons with phosphorus tribromide and subjecting the admixture thus obtained to an extractive distillation.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE EXTRACTIVE DISTILLATION SEPARATION OF CLOSE-BOILING AROMATIC HYDROCARBONS WHICH COMPRISES ADMIXING WITH SAID HYDROCARBONS AS AN EXTRACTIVE DISTILLATION ENTRAINER PHOSPHORUS TRIBROMIDE AND THEN SUBJECTING THE ADMIXTURE TO DISTILLATION. 